The purpose of this study is to examine the effects of UV light on auto-immune strains of mice. Humans with systemic lupus erythematosus often have worsening of disease after exposure to sun light. Three strains of auto-immune mice, BXSB, (NZBxBZW)F1 and MRL mice were exposed to either acute or chronic doses of UV light. It was observed that male BXSB mice had increased mortality after such treatment. They also had increases in levels of antibodies to ssDNA, increased antibodies to the TNP hapten and increased TNP-plaque forming cells in their spleen. Thus their B cells appeared activated. Sections of kidneys of male BXSB mice demonstrated increased inflammatory cells in their glomeruli. BXSB mice appear particularly susceptible to the deleterious effects of UV light. Our studies give a partial explanation for the deleterious effects of UV light in human SLE.